spe 26453 cement resistivity

8/2/2019 SPE 26453 Cement Resistivity

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SPE 26453Cement Resistiwty and Implications for Measurement of FormationResistivity Through CasingJ.D, Klein, P.R. Marln, and A.E. Miller,* ARCO E&PTechnologyqSPEMember

Socletv of PetroleumEndmtra

Copyright 1993. Society of Petroleum Engmoers, Inc.This papa{ waeprepared for presentation at the 68th Annual Technical Conference end Exhlbdlon of the Society of Pelroleum Engmeere held In Horrslon,Texes. 3-S October 1993.Thta paper wasaalected for presentation by an SPE Program (%mmmee followlng rewaw of information contained m an af)strecl subm!lled by the author(e). Contents of the paper,aa preaentad, have nof osen reviewad by the Sociely of Petroteum Engineers and are eubject to co rrec ti on by me au thor (s) Thematerial, as presented, does not necessarily reflectany position of Ihe Society ot PetroleumEnginesra, iteofficers, or mambere.Papers presentad at SPE msellnge ere subjact 10publlcaoon rewewby EdoorialCommineee01the SocIeIyof PetroleumEngineere. Permieeiontocopy is restrictedto an abstractof notmorelhan 300words.Illuatrallonsmeynot becopied. TheebaheclshouldC4nleinconaplcuoueacknowledgmentof where and by whom the paper ia presented. Write Librarian. SPE, P.O. Box 833838, Richardson, TX 75083.3836, U.S.A. Telex, 163245 SPEUT.~CTMeasurementof formation resistivity through casing is anemerginglogging technology, with prototype loggingtoolscurrently under development, Modelingstudies showthatthe presenceof cement could reduce the sensitivity of theTCRTW (Through Casing ResistivityToolTJ). Physicallythis tool has some similarityto a Iaterolog. Measurementsvvmddbe seriously affected if the cementwas muchmoreresistive than the formation. We have carried out alaboratory studyof cement resistivitywith the funding fromthe Gas Research Institute. Our main objective was toprovideharddata to use \nassessingthe impactof cementresistivityon measurementsmadewith the TCRTTM.Our results show that cement resistivity is generally low,varyingfrom less than 1 to 8 Q-m at 120F. Light-weightcementswere generally the least resistive,with ClassA, Gand l-fcementsmore resistive. Resistivitywas essentiallyindependent of either confining or pore pressure. It wasalso independent of either current density or voltage.Cement resistivity appears to vary with temperatureaccording to Arps equation, as would be expected fornonconductivematerial saturatedwithwater.Thinsectionsand mercuty injectioncapillary pressuredatashow that porosity in cement is dominantly microporosity,with pore throat diameter less than 0.1 microns. Inaddition helium porosity is high, ranging from 3!5to 40%.The low obsewed resistivity is due to large amounts ofwater-filledmicroprosity.

The results of this study signify that the presence ofcement will not seriously degrade measurements ofresistivity through casing in many environments. Ourwork concentratedon cements normallyused on the NorthSlope of Alaska, as well as Class H cement, a commonGulfCoastcement.Potential applicationsof the TCRTTMinclude water floodmonitoring at Prudhoe Bay and elsewhere, and detectionof missedpay behindcasing.

UcmRecent technological advances irrcficatethe feasibility ofmeasurement of formation resistivity through casing.Patents on the basic technology are held by Gard et al.(1989), Kaufman (1989), and Vail (1989a, 1989b, 1991a,1991b, 1991c, 1993a,and 1993b). The concept has beendemonstrated by successful measurement of formationresistlvityin a casedwell (Vail etal,1993). Vail isCUrrentlydevelopinga prototype logging tool.The basic conceptsfor the measurementaredescribedbyKaufman (1990). Early in the development of thetechnology it was recognized that a high resistivitycementannulus or high casing contact resistancecould affect themeasurement of formation resistivlty. The tool propxedby Vail operates similarly to a Iaterolog, and is thereforesensitive to the presenceof large resistancesbetween thecasingand the formation. The effectof a cementannulus

Referencesand illustrationsat end of paper. TMThroughCasing ResistivityToo! and TCRT aretrademarksof ParaMagneticLogging, Inc.365


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2 CEMENTF?ESISTWITYAND lMPLiCATIONSFOR MEASUREMENT SPE26453OF FORMATIONRESiSTIVITYTHROUGHCASINGhas been described theoretically by Morrison aridSchenkei (1992), and is the subject of an ongoing study atthe University of Houston (Shen, 1992). The theoreticalstudies are hampered by a paucity of published data oncement resistivity. The present study was carried out toprovide information on the resistivity of cementformulations that are commonly used by the oil industry tocement casing. The specific cements studied and the testconditions pertain generally to the North Slope of Alaskaand the U.S. Gulf Coast, but the general conclusionsshouldapply elsewhere.The through-casing resistivitymeasurementscould alsobeaffected hy large casing contact resistance. A secondaryobjective of thjs study was to determine the effectof casingcontact resistance using electrodes composed of carbonsteel,~The study Includedpreliminarymeasurementsdesigned tobenchmark the system repeatability. Thesemeasurements documented the repeatability of thehardware;the repeatabilityof the measurementfor multiplesamples of the same batch of cement; and finally thevariability between cements for four different Class Gmanufacturers.A number of additional measurementswere carried out toisolate and identify any measurement or preparationartifacts that might affect the data. The following variableswere included:

1) cored versusmoldedsamples,2) measurement frequency and current density,and3) curing conditions (pressure,temperature, and mixwater).The first item was included to determine if coring ofmeasurementplugs would cause microscopicfracturing ofthe cement that could lower the measured resistivity. Thesecond item was considered important since the toolproposedby Vail operates at 1 to 2 Hz, but the equipmentused in the ARCO electrical rock properties laboratoryoperates at 10 Hz and above. It was desked to obtaindata at .;te same frequency as the tool operates. Henceequipmentwas rentedfor low frequency measurements.Our main effort was to study the resistivity of cement, andto seek any dependenceon the following variables:

1) porepressure,2) sleeve (or overburden) pressure,3) temperature,4) cement type and source, and5) cement age.

During the study we also measured various cementphysjcal properties, including porosity, grain density, andmercury injection capillary pressure. Generall~, thecement permeabllity was too low to measure. Finally,west~died casing contact impedance, including itsdependence on current density, frequency, and biasvoltage. Forsome t~sts,electrodeswere constructedfromcarbonsteel, preparedas either shiny or rusty.BO~TOFtY E~T ANfI PF?O~

Electronic EquipmentThe measurement system used normally in the ARCOelectrical laboratory is set up to automatically measureresistkity usjng either two or four electrodes over thefrequency range of 10to 100,000Hz.Sincedatawere requiredat lower frequency than could beobtained with this gear, we also used some generalpurpose geophysical equipment that operates over thefrequency range of 0.001 to 8,000 Hz. This equipmentwas used for four-electrode measurements, and hadmaximum current output of 100mA, needed to study theeffects of current density. The system has automatic gainsetting and 16 bit resolution. The low frequencyequipment has special purpose software for laboratorymeasurementson core samples, and so was ideallysuitedfor the study.Sample Holders and ElectrodesThe sample holders are designed for either two- or four-termlnalmeasurements,as illustrated in Figure 1, Mostofour measurementswere obtainedwith silver mesh currentelectrodes. For the study of casing effects the currentelectrodes were constructed of casing steel. The inner(potential electrodes) are composed of silver wire thatencircie the core at positions of one-third and two-thhdsalong its length. The electrode cell is designed so thatfiuid can be flowed through it to control pore pressureandfor displacement experiments,and sleevepressurecan beapplied to simulate overburden pressure. Most of theexperimentswere carried out with 500 psi pore and 1,500psi sleeve pressure. Pressure is necessary to force thecurrent and potential electrodestight against the sampletomaintaingood electrical contact. In addition to applicationof pore and sleevepressure, the electrode cell is placed inanoven for temperaturecontrol.Laboratory resistivity measurements can be carried outwhh either two or four electrodes. A good overviewof themethodologyis given byWorthington et al. (1990).Figure1 shows an equivalent electrical circuit that describes themeasurements. In two-electrode measurements thefunctions of the current and the potential electrodes are. . .comkjned together. This approach has the advantages01

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SPE264@ J. D,KLEIN,P.R,MARTIN,A, E.MILLER 3simplicityand provides data for the entire core plug, as theelectrode$ are mounted at the ends of the plug. Two-electrode measurements are valid provided electrodepolarization associated with the electrode-samplecontactresistance is rnlnimjzed. This is done by carrying out themeasurements at high frequency and by increasing theelectrode surface area. However, electrode polarizationbehaves as a capacitor, with its impedance increasingrapidly with decreasing frequency, as illustrated in Figure1. Thus two-electrode data am invalid at low frequencywhere the electrode responsedominatss.Four-electrodemeasurementsentail separatecurrent andpotential electrodes, and thus avoid the voltage dropacross the current electrodes in the measurement. Thepotential electrodes contact the sample at two locationsalong its length, using circumferential wires. Themeasurement determines the reslstlvity of the volume ofrock between the potential electrodes, and samples asmallervolumecomparedto two-electrodemeasurements.Four-electrode measurements accurately determineresistivity at low frequency, as illustrated in Figure 1. Atvery high frequency, above approximately30,000 Hz, bothtwo and four-electrode data can be affected by straycapacitanceof the hardwareandconnectingwires.The electrode polarizationobservedwith the two-electrodemeasurement provides an ideal means of studying theresponse of casing surface impedance, The silver screenelectrodes normally used were replaced by perforatedcarbon steel disks placed in contact with the cement. Thecell resistivity was then measured versus frequency as afunction of current density, current history, and surfacecondition of the electrodes.Cement Sample PreparationCements samples were prepared using API mix waterrequirements with the mixing procedures outlined in APISPEC. 10, Section 5. The slurries were transferred intostandard brass compressive strength molds. The mo!dswere lightly greased to facilitate ease of removing thecement cubes without damaging the cement samples.Following slurry placement into the molds, the slurry waspaddIed 25 strokes with a glass rod to eliminate any akentrained in the slurry matrix. Each mold contained twocubes fromwhich cylindricalcore were drilled and faced tofit the Hassler Wton sleeve for resistivlty measurements.The cement plugs were 1.5 inch in diameter and 2,0 inchin length. Ambient cements were submerged underwaterand cured for seven days. Reservoir-condition cementswere loaded into a curing chamber, and cured at 1500Fand 3,000 psi for seven days.A varjety of different cement formulations and vendorshave been used in this study. ClassG cement mixedwithfresh water has been used at Prudhoe Bay below thepermafrost. Class G cement mixed with 18% NaCl by

weight of water has been used above the permafrost.Latex cement is presently being used at f%udhoe Bay.Hence two latex formulations were included here, alongwith several other Arctic formulations. Class 1-fcements,which are common m the U.S. Gulf Coast, were alsoincludedin the study.!!IJ!IY REME! EATAt31LPreliminary studies were carried out to establishrepeatability for the measurement system and to obtainsome initial cement data prior to the main investigation.The study was designed to verify reproducibility of thefollowing:

1] measurementsystem, including the four Identicaltest cells,2) resistivitywithin a single batch of cement, and3) resistlvity between cements of the sametype fromdifferentvendors.The sampleswere 1,5 inch in diameter and 2.0 inches Inlength. They were mounted in the 4-electrode cells, andsleeve and pore pressures of 1,500 and 500 psirespectivelywere applied, The cells were placed in an ahbath at 72 to 73F, and either water or brine was flowedinto the sample.hleasurlng System ReproducibilityThe repeatability of four-electrode measurements usingfour identlcai test cells from the same manufacturerwasdetermined using one Berea sandstone core plug. Thesample was mounted in each test cell in turn, and thenbrine composed of 2 percent-by-weight NaCl (Rw = 0.30Q-m at 72 F)was flowed into the sample to saturate theplug. The core plug reslstivltyat 720 Fwas monitoreduntilstablewith time, and the resistivity and phase angle weremeasuredusing the high frequency equipment from 11 to100,000 Hz. The mean reslstivity for the Berea samplemeasuredin four different cells at 11Hzwas 6,29Q-m (at72 F), with standard deviation of 0.03 f2-m,equivalenttoa relativeerror of 0.49Y0. At 1,000 Hz the meanwas 6.26Q-m (at 72 F), with the same standard deviation. Asshown in Figure2, the resistivity and the repeatabilitytendto decrease for frequency above 10,000 Hz. These aresystematic errors associated with the measurementsystem itself (electrode polarization and straycapacitance). This set of measurements demonstratesthat the measurement system, including electrode cells,repeats to within 0.50% for frequency less than 1,000 Hzand to within 1.0% for frequency between 1,000 and10,000 Hz. The relative error increases rapidly atfrequency greater than 10,000 Hz, due primarily to thedata obtained with Cell #2. This cell has anomalous

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4 CEMENTi3ESlSTlVlTYAND IMPLICATK3NSF(3RMEASUREMENT SPE26453OF FORMATIONRESISTIWTYTHROUGHCASINGbehavior at high frequency, perhaps due to greatercapacitance.Cement Batch ReproduclbtlityThree plugs were cut from one batch of Vendor 1 Class Gcementpreparedas shown InTable 1, Samples 1 through3. Deionized water was flowed into each sample toreplace any fluids lost during coI ing and mounting.Injection was continued until sufficient effluent wasobtained for determination of Rw, but not long enough toachieve equilibrium. Thus the cement piug resistivity andthe effluent resistivity increased very slowly with time.Throughputwas too S!OWo attempt to equilibrate the porefluids. Resistivities of the three plugs were determinedusing both the high frequency gear (1O to 100,000 Hz)and the low frequency equipment (0.50 to 128 Hz), andwith four-electrodemeasurements.Little difference in resistivity at 1200 F was observed forthe three cement piugs. The variability between sampleswas small, within 6%. In addition the two measuringsystems repeat to within 1?40, When overlappingfrequencies are compared, the iow frequency valuesaverage6.33~0.05 Q-m from8 to 128 Hz. This comparesto 6.34*0.04 Q-m for the high frequency gear over thefrequency range 10 to 100 Hz. Merged resistivity data forthe three samples areplotted versusfrequency in Figure3.The slight mismatch between two sets of equipment isapparent in the data at the overlappingfrequencies.Class G Cement ReproducibilityThe finai set of preliminary measurements determinedreproducibility between four different Ciass G cements.One plugwas obtained from each of the cements listed inTable 1 as Sampies 4 through 7. A 2% by weight NaCibrine was flowed through each sampie to repiace iostfluids, but not iong enough to achieve equilibrium. Thepermeabilities of the core piugs were too iow to aiiowdisplacement experiments to be compieted. We did notattempt to determine initiai resistivities by extrapolating tozero fiuid throughput. Four-electrode resistivity wasdeterminedat 120Fwith the high frequencygear from 11to 100,000 Hz. Data for the four samples are shown inFigure 4. The variation in resistivity between the fourvendors is approximately 40%, ranging from 3.47 to 5.26O-m.

iSTiViTVinfiuence of Cement Sampie Preparation

plugs, This would be of particular concern if the cementplugs were highly resistive, since surface fracturing mightcause the measured resistivity to be iess than the actuaicement resistivity. Cylindrical samples were prepared bypouring the cement siurry into 1.5 inch diameter molds.Two molded piugs were prepared. For comparison, twocored piugs were driiied from cubes prepared from thesame batch of cement siurry. The cement was preparedwith tap water and cured at ambient conditions for 7 days.For this study resistivity was measured using the highfrequency gear and two electrodes. Tabie 1 reports theresufts,for Sampies 12through 15. The data cieariy showthat the method of preparation does not have a majorimpact on resistivity, with the molded sampies havingsmailer resistivity than the plugged samples. This isopposite to what was expected, but is perhaps due tosurlace fracturing aiong with some drying of the plugs,whichwere not flowed through. Surfacedryingwouid tendto increase their resistivity. We conciude that coring thecement piugs may cause measured resistivities to beiarger, not smaiierthan the intrinsiccement resistivity.Samples 8 through 11 in Tabie 1 were used to study theeffects of differences in the method of preparation. Twosampieswere cured at ambient conditions of temperatureand pressure,and two were cured for sevendaysat 1500Fand 3,000 psi. The mix water of the sampies was aisovaried, with tap water and 18% NaCl being used, Theresistivities of the sampies varied from 2,4 to 5.2 Cl-m.Sample 11, cured at ambient conditions and mixed withIWO NaCi, was anomalous,with higher resistivitythan theother 189oNaCi sampie. Other data, not reported here,ciearly show that samples preparedwith 18?40NaCi rangein resistivity from 1.7 to 2.1 K&m. The saiinity of the mixwater is important,with lower resistivityobtainedwith highsaiinity. ComparingSamples 8 and 9 suggests that curingpressuredoes not significantlyaffect resistivity.Dependence on Measurement FrequencyCement resistivity was measured at the same frequencyasproposedfor the operation of the tooi. Thiswas done inorder to ensure that representativedata were obtained. Atissue is the possiblefrequencydependence of the cementresistivity, Data shown in Figures 3 and 4 show a weakfrequencydependence for frequency above approximately100to 1,000 Hz. For lowerfrequncy, and for this cementtype there does not appear to be any significant frequencydependence. The cement resistivity observed at 10Hz isneariythe sameas that obsewed at 1 Hz. Becauseof theminimal dependence of cement resistivity on frequency,muchof the remaining cement data were measuredwiththe high frequency gear and are reporled at either 10 Hz(four-electrode measurements] or 20,000 Hz (two-eiectrodemeasurements).During the initiai stages of the study there was concernthat coring the cement plugs from the 2 inch cubes wouidcause fracturing at the surlace aiong the iength of the

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SPE26453 J. D. KLEIN, P. R, MARTIN,A. E. MILLER 5

Dependence on Current DensityA final set of measurements determined if cementresistivity depended cm current density of themeasurement, Four-electrode data were obtained usingihe low frequency equipment over the range of currentdensity from approximately 0.007 to .7 mA/cm2 Themeasurementswere carried out on Samples 8 through 11.The results show that cement resistivlty varies less than2% and is essentiallyindependent of current density.Dependence on Pressure and TemperatureWemeasured resistivityof four different plugs of Vendor 1Class G cement prepared as shown in Table 1,Samples8through 11. These studies determinedthe dependence ofresistivity on pore pressure, sleeve pressure, andtemperature. Figure5 showsthe dependence on pressurefor Sample 80 Essentially no variation in resistivity wasnotedfor overburdenpressun?rangingfrom 1,000to 4,000psi (with pore pressure at 500 psi), or for pore pressureranging from 1,000 to 3,500 psi (with overburden pressureat 4,000 psi). The insensitivity to pressure for thisparticularcement indicatesthat it is incompressible.Fjgure 6 shows change m resistivity with change intemperature for Sample 32. The dependence ontemperature is consistent with a change in brineconductivity with temperature (F), as expressed by ArpssEquation:R2= RI (TI + 6.77)/(f,T2+ 6.77) . . . . . . . . . . Equation1The discrete points in Figure 6 are observed data. Thesmooth line is from Equation 1. These results indicatethatresistivity measured at one temperature can be correctedto what would be measuredat another temperature.Dependence on Cement TypeWe measured resistivity of a variety of different cementtypes, as fisted in Tabtes 1 and 2, Samptes 16through 35.The list included a number of different cements, includinglatex and fly ashforr ?tions. Figure6 showsa histogramof the results, showing resistivities corrected to 1200 Ffrom measurement temperature using Equation 1. Datafor Samples28 and 29were not included in the figure.The Arctic and light weight formulations have the lowestresistivity, with the exception of Class H samples withcenospheres. Cenospheres are used uncommonly toachieve very low density, but expensive cements,Cenospheresarehollow glassspheres filledwith air, whichexplains why they do not lower the cement resistivity.Light weight samples with resistivity less than 1 Q-mpresumably contain a much higher amount ofmicroporosity compared to the other samples, although

this has not been confirmed. The series of samples 30through 35 show increasing resistivity with increasingcement density, due perhaps to high density additivesthatare non porous, The effect of latex added to Class t-fcementwas ambiguous,with slight diffeiences in reslstivityobse.veal. Latex did not cause any significant increase inreslstivity, Figure 7 clearly shows that all of the cementtypes tested have resistivity less than 8 Q-m at 1200 ,with the exceptjonof the cenospheresas noted above.Dlffuslon EffectsAn issue that has not yet been addressed is the degreetowhich ions are exchanged between the cement andadjacentformationwater. Ifthe bulk of the water is boundwater, then it would be expected that ion exchangewouldbe low. Cement resistivity would then reflect the initialcomposhjon of the cement mix, plus effects of cementaging. On the other hand, if there was a significantfractionof free water or capillary water, then ion exchange mighttake place via diffusion. Over time cement would thentend to equilibratewith the formation brine. An experimentwas carried out in order to identify possible ion exchangebetweencement and surroundingfluids.Three cement plugs (Samples25,26, and 27) were storedin 3% brine at 150 F and ambient pressure. They weretemporarily removed from the brine and their reslstivitymeasured at regular intervals over a period of severalmonths. Sincethis was the samesalinityas the mixwater,this experiment was an attempt to determine long termchanges in the cement Itself that might be due torecrystallizationof the matrix, and associated changes inthe pore structure. Fjgure8 shows a decreaseIn resistivityof these samples with time. Either the pore structure ofthese samples is changing or there is net movement ofions into the cement from the brfne.Two cement plugs were stored in a container of initiallydistilled water at 150F and ambient pressure in order todetect net diffusion of ions out of the cement. Figure 8shows that the plugs increased in reslstivitywith timeovera 30 day period followed by a smalldecrease in resistivity.These data suggest that Ionsdiffuse out of the cement intothe surrounding distil led water, followed by a long termdecrease in resistivity due possibly to dissolution of thecement.GA~lNG EFFFCTSAs described in an earlier section, two-electrodemeasurementsare affected by ihe voltagedrop associatedwith the electrode-sample contact. The electricalimpedance of this contact is controlled by the interfacebetweenthe metalof the electrodeand the brinecontainedin the cement that contacts the electrode. The impedance

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6 CEMENT RESISTIVITYAND IMPLICATIONSFOR MEASUREMENT SPE26453OF FORMATIONRESISTIVITYTHROUGHCASINGof metal electrode-brine interfaces has been studied indetail by electrochemlsts. It depends on a variety offactors. Interface impedance decreases as frequencyincreases and as the exposed area of the electrodeincreases. The interface impedance also depends on thenatureof electrochemicalreactions by which current flowsacross the interface. Thus if the reactants are present inhighconcentration, and If the reaction can proceed easily,then the Interface impedance will be negligible.Alternately, if the reactants are scarce or if the reactionrequires high activation energy, then the interfaceimpedancewill be quite high. In this case the frequencydependent capacitance of the electrode will dominate itsresponse, as illustrated in Figure 1. Electrode impedanceis also a strong function of current density. Increasingthecurrent density puts energy into the system to activatecharge transfer reactions, and thus lowers the interfaceimpedance. Application of a bias offset voltage willnormally have the same effect. Thus, a high impedanceinterface will display a strong dependence on currentdensity and offset vottageas well as frequency.Casingpresent in the earth behaves as an electrode,sincecurrentmustpass from the tool through the casing intotheformation, where current flows by rnovenlent of ions insolution. Thus measurements madewith a two-electrodecell and using casing steel as electrodes should provide adirect indication of casing effects. l-fence a set ofmeasurements were carried out using casing steel forelectrodes. Datawere obtained as a function of frequencyandcurrent density for both shiny and rusty electrodes.Shiny ElectrodesFigure 9 shows resistivity versus frequency for two-electrode measurements made with shiny carbon steelelectrodes. Curves are shown for the sequence ofcurrents shown in the figure, which started out low,Increased to a high value and then repeated with a lowvalue at the end. The initial data at low current showstrong frequency effects related to extremely high surfaceimpedance of the electrodes. At high frequency theelectrode impedance becomes very small, and themeasurementIs dominatedby the resistivityof the sampleitself. As current tevel was increased, the increasedvottagescause a decrease in the interface impedance anda reduction in the measured resistivities. Note that oncethe interface was subjected to high current it changesirreversibly, so that the low-current repeat data isessentially the same as the high current data, and largelyreflect resistivity of the cement plug. Results similar tothese were obtained by application of a strong offsetvoltage,again startingwith freshly polishedelectrodes.Rusty ElectrodesThe next set of measurements were made using carbonsteel electrodes that had been allowed to rust. This was

done by repeated cycles of wetting and dryingwith a highsalinity brine. Figure 10 shows that the lusty electrodeshave no influence on the measurement of resistivity, asevidenced by the lack of dependence on either frequencyor current density. We attempted but failed to consttuctsampleswith the cement bonded to rusty electrodes. Tofurther study rusty electrodes we measured the etectrodecell filled with a conductive brine to reduce the sampleresistivity. The results for both shiny and rusty electrodesare shown in Figure 11. The polished electrode at lowcurrent density clearly displays strong electrodepolarization, as expected. The rusty electrode has noinfluence on the results, and must therefore be extremelyconductive. This data clearly show that rusty casing hasextremely low sudace impedance, and thus should notaffect the operationof a through-casing resistivltydevice.

T PHYSICALPWERTIE.SHelium porosity and grain density were determined onSamples 8 through 11. The results are included in Table1. The high measured porosities are present asmicroporosity, as is evident in the mercury injectioncapillary pressure data in Figure 12. This data wereobtained to quantify the amount of microporosity and toevaluate pore size distribution, Prior to mercury injectionthe plugwas oven dried at 1050C. The air permeabilityofthe sample was also determined to be 0.261 md. Duetothe low permeability, the sample was given extendedequilibrium times during mercury injection. Injection wascarried out up to 30,000 psi air /mercury with a drainagecycle only. Figure 12 shows the capillary pressure curvealongwith the pore throat radius. This data showthat over95% of the pore throats have radii less than 0.10 micron.The cement porosity is thus totally dominated bymicroporosity. Water in the cement is thus either boundwater or water held by capillarity, with little or no freewater.

The resistivityof water-saturatedcements at 1200F rangesfrom less than 1 to approximately 8 Q-m. There appearsto be a bimodal distribution. Class G, A, and Hformulations have resistivities in the range of 4 to 8 Q-m.Latex additives have fittle or no effect on resistivity. Thefightweight and Arctic formulations generally havereslstivities less than 1 Q-m, with the exception of thecenosphere additives, which cause anomalously largeresistivity. The resistivity of cement is controlled by thehigh porosity, which is dominantly mlcroporosity. Althoughcement mix water salinity is a factor, we believe that theresistivity of the cement is determined by the resistivityofthe water contained in the pore space at the time of

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SPE26453 J. D.KLEIN,P.R. MARTIN,A. E.MILLER 7measurement. This is cflfficultto demonstrate due to thelow cement permeability. Cement resistivity appears tovarywith temperature according to Arps Equation.Cement reslstivlty is essentially independentof overburdenand pore pressure, reflecting a high compressive strength.This conclusion may not apply to the Arctic formulationswtdch tend to remain deformable, even after curing,Resistivlty does not depend on current density, but isweakly dependenton measurementfrequency.Laboratory studies show that rusty casing hasexceptionally low Impedance, and will not constitute aproblem for the operation of the Through-CasingResistivityToolTM Lacquered casingwas not tested, butcould pose problems for the operation of the tool.Diftuslon experiments suggest movement of ions betweencement and adjacent brines. Because cement has highporosity,we expect it will usually have lower resistivitythananywater-bearingformation that h contactsand withwhichit has reached ionic equilibrium. The results describedhere show that cement resistivity will not Interfere withoperation of through casing reslstivity devices under mostconditions,

Richard Jones and Kim Jacobs of AEPT prepared thecement samples and advised us on cement properties. C.L. Vavra also of AEPT supervised the mercury injectioncapillary pressure experiment. Mark Alberty of BritishPetroleum provided input on cement type and overalldesign of the study, as did Jake Rathmeli of AEPT. JackOstrander assisted with the electrical propertiesmeasurementsB~sGard, M. F., Kingman, J. E. E., and Klein, J. D., 1989,Method and apparatus for measuring the electricalresistivity of geological formations through metal drill pipeor casing: U.S. patent 4,83:,518.Kaufman, A, A., 1989, Conductivity determination in aformation havinga cased well: U.S. patent 4,796,186.Kaufman, A. A., 1990, The electrical field in a boreholewith a casing: Geophysics, v. 55, p. 29-38.Morrison, ii. F., and Schenkel, C. J., 1991, Numericalanalysisof d. c. logging through metal casing: Final reportto ParaMagnetic Logging, Inc., under Contract No. UCBEng-7724funded by the Gas ResearchInstitute.

Shen, L., 1992, Well Logging Laboratory, DepartmentofElectricalEngineering,Universityof Houston.Vail Ill, W. B., 1989a, Methods and apparatus formeasurement of the resistivity of geological formationsfromwithin cased boreholes: U.S. patent 4,820,989,April11.Vail Ill, W. B., 1989b, Methods and apparatus formeasurement of electronic properties of geologicalformations through borehole casing: U. S. patent4,882,542, November21.Vail Ill, W. B., 1991a, Methods and apparatus formeasurement of the resistivity of geological formationsfrom within cased wells kr presence of acoustic andmagnetic energy sources: U.S. Patent 5,043,669, August27.Vail Ill, W. B., 1991b, Methods and apparatus formeasurement of electronic properties of geologicalformations through borehole casing: U.S. Patent5,043,668,August 27.Vail Ill, W. B., 1991c, Electronic measurement apparatusmovable in a cased borehole and compensating for casingresistancedifferences: U.S. Patent 5,075,626, December24,Vail Ill, W. B,, 1993a, Measuring reslstivity changes fromwithin a first cased well to monitor fluids hjected into oilbearing geological formations from a second cased wellwhile passing electrical current between the two casedwells: U.S. Patent5,187,440, February16.Vail Ill, W. B., 1993b, Methods of operation of apparatusmeasudng formation resistivity from within a cased wellhavingone measurementand twocompensationsteps: U.S. Patent5,223,794, June 29.Vail, W. B., Momii, S. T,, Woodhouse, R., Alberty, M.,Peveraro, R. C. A., and Klein, J. D., 1993, Formationresistivity measurements through metal casing:CWLS/SPWLA Joint Formation Evaluation Symposium,June 13-16.Worthington, P. F., Evans, R. J., Klein, J. O., Walis, J. D.,and White, G,, 1990, SCA Guidelines for samplepreparation and porosity measurement of electricalresistivity samtXes: Part Ill - The mechanics of electricalresistivity measurement on rock samples: The LogAnalyst,v. 31, p. 64-67.

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CEMENTRESISTIVITYAND IMPLICATIONS FOR MEASUREMENT SPEOF FORMATION RESISTIVITY THROUGH CASINGTABLE 1

Sample Preparation and Resistivity,ample Cement Curing Mix Flowlumber Resistivity-r&P Water Water Frequencyven~:~l 1500F tap water de ionized 6,58 n-m3,000 psi 1 Hz~:~dso&l 150 tapwater de ionized 6.283,000 1

I Vendor 1 150 tapwater de ionized 6,17Class G 3,000 1I Vendor1 150 tapwater 2%NaCl 4.3ClassG 3,000 11i Vendor 2 150 tapwater 2%NaCl 5.4ClassG 3,000 11; Vendor3 150 tapwater 2%NaCl :i6

ClassG 3,000v Vendor4 150 tap water 2%NaCl 4.6ClassG 3,000 11 T) Vendor 1 150 tapwater 2%NaCl 4.88 $=38.7ClassG 3,000 1 gd=2.27) Vendor1 ambient tapwater 2%NaCl 4,55 4=37.4ClassG 1 gd=2.2810 Vendor1 150 18% NaCl 2% NaClClass G 2,37 Q=40.13,000 1 gd=2.3411 Vendor1 ambient 18% Nacl 2?4NaClClass G 5,18 Q=36.71 gd=2,2712 Vendor 1 ambient tap water 4.85Class G molded 20 kHz13 Vendor 1 ambient t;:l~:$w 4.35

Class G 20 kHz14 VJ:Klrl&l ambient $o~e~ter 5.6320 kHz15 Vendor 1 ambient :o:e~ter 5.94Class G 20 kHz16 Vendor 5 150 tapwaterClassA 7.$&72F3,00017 Vendor5 150 tapwaterClassH 3,000 11.23@72F10HZ18 Vendor 5 150 tapwaterLightweight 3,000 0.79@72F10Hz19 Vendor 5 150 ta:) waterPolyvinyl 12.33 @72F3,000 10Hz20 Class H plus 150 tap water

Styrene resin10.9 @ 72F

3,000 10Hz 121 Class G ambient tap water 1,90 @72F+ 8Yegel 10Hz22 ambient tap waterBJ Coldset 3 0.37 @72F10HZ23 ambient tap waterPermafrost E 0.28 @72F10Hz24 ambient tapwater 0.36@72FI Arctic Set 3 I I I I1o Hz I

Resistlvitydata at 500 psi pore pressure,1,500psi sleevepressure,and 120F unlessotherwisenoted. The symbols $ and gd refer to porosityand grain density respectively.Piano tap water has resistivityof 66Q-m at 68 OF.Sample 19 isClassH plus latex (polyvinylafcohol1.5-2Yo)372

0 .

26453


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J. D. KLEIN,P.R. MARTIN,A. E.M;LLER 9

*,,

SPE26453

TABLE 2Class H Cements

Reslstkity for Different Additives and WeightsSampleNumber25 none 2.50 S&m26 1% polymer 2.5527 35% silica flour 1.9328 cenospheres Ioppg 30.5029 cenospheres 12 33.9530 gel 14 (MO31 gel or sodium metasilicate 15 1.16 ,32 none 16 2.2833 dispersant 17 2.9834 hematite 18 3.8035 hematite 20 4.43

Cements preparedwith 30/0NaCImixwater, andmeasuredat 20,000Hz, 150Fand 2,000 psi sleevepressure.

p73


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10 CEMENTRIXNSTIVITYAND IMPLICATIONSFORMEASUREMENTOF FORMATIONRESISTIVITYTHR0LN2J-1ASING

TWO ELECTRODESOURCE VOLTAGE

-re core plugL-1JLECTRODE zIMPEDANCE ew4 FOURE~EOTR()~EVOLTAGEFigure 1. Diagrams showing equivalent circuit and associated plot of reslstlvityversus frequency for two- and four-electrode measurements. For two-electrode measurements the current and potential electrodes arecombined, and the measurement includes frequency-dependent voltagedue to electrode surface impedance. This problem is avoided with four-electrode measurements.

374,

\FOURELECTRODE

TWO ELECTRODERESPONSE

RESPONSEJ

FREQUENCY


11/16

*

SPE 26453

!5

Figure 2.

Figure 3.

J. D.KLEIN,P.R. MILLER,A. E.MILLER 11

A Cell#lA Celi#2--9- Cell#3_ Celi#4

10 160 1000 1OQOO 100000FREQUENCY (HZ)

Flesistivity versus frequency for one Berea core plug mounted in fouridentical core holders showing measurement repeatability.

7

5 00FREQUENCY (HZ)

Resistivity versus frequency for three different core plugs from the samebatch of Class G Cement, showing repeatability between plugs, andbetween the. high and low frequency sets of equipment. The arrowsatthe top of the figure show the overlap in frequency for the two sets ofequipment.375


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12

5,5

CEMENTRESISTIVITYAND IMPLICATIONSFORMEASUREMENTOF FORMATIONRESISTIVITYTHROUGHCASING

4567

SampleSampleSampleSample

10 100 1000 10000 100000FREQUENCY (HZ)

Figure 4. Flesistivity versus frequency for Class G cements from fourvendors, showing the variability between cements.

43.53

2.5Figure 5.

different

C30go?- F

-1 ovERmRDm PORE PRESSLJREPRESSURE (overburden pressure(pore pressure fixed at 4,000 psi)fixed at 500 mi)J

Plots of resistivity showing the effects of changing pore pressure andsleeve pressure. Cement resistivity is essentially independent ofthese two parameters.


13/16

qI

E 26453 J. D.KLEIN,P, R,MILLER,A, E,MILLER 13

Figure &

00 120 140 160 180 200TEMPERATURE (DEG F)Plot of resistivity versus temperature for cement sample 32. The line isresistivity as predicted by Arps equation, Equation 1. Cementresistivity appears to be controlled by the temperature of the containedbrine.

10987UfjLu55

~43210

1LIGHTWEIGHT& ARCTIC/ CEMENTSh. CLASSESA, G, AND H D,

I 1012345678 9 10

RESISTIVITY (OHM-M)Figure 7. Histogram of resistivity for different cement types. Measurementconditions vary, but all data are reported at 1200 F. Data for Class Hcement with cenospheres are not shown.

377


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CEMENTRESISTIVITYAND 9MPLlCAT10NSFORMEASUREMENT SPE 26453(3FFORMATIONRESISTIVITYTHROUGEICASING

o~o 20 40 60 80 100TIME (DAYS)

7A6:5; #< + PLUG 314321 SAMPLES INo~ n #1 I 11 91 {o 20 40 60 80 100TIME (DAYS)

Figure 8. Plots of resistivity versus time showing change in resistivity as sampleequilibrates with surrounding fluids, demonstrating the exchange ofions between cement and external brine.

Figure 9.

1c

1,OW CURREl JT

A 9.49 UA+ 50 UA--m-- 498 UA-A-- 4.97 mAs. 10.35 mA- 7.84 UA repeat

HIGH CURRENT

I I I l--nII b I I 11111 1 I 111111 I 1 I I Il(r1 10 100 1000FREQUENCY (HZ)

De~endence of two-electrode cement resistivity on frequency andcu~rent for oolished steel electrodes. Electrode effects observed atlow current level disappear irreversibly at high current levels.378


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10

1

J. D.KLEIN,P.R. MILLER,A. E.MILLER

. ,lllnln

m m q

1 I 1 I Ill0.1 1

I1--a= 8.53 UAQ 505 UA-m-- 10.2 mA

m

II i I 111111 1 I I 111() 100 1(

16

0FREQUENCY (HZ)

Figure 10. Dependence of two-electrode cement resistivity on frequency andcurrent level for rusty steel electrodes. The complete absence ofelectrode response indicates casing will have no effect on theoperation of the Through-Casing-Resistivity-Tool.

33$ =m...- rusty electrodesg 1z

~ polished electrodes~t-g0 -wa

0.1 f I 11[111 I I I 1111 1 i 111111 1 I I IIln0.1 1 10 100 1(FREQUENCY (HZ) oFigure 11. Two-electrode data obtained with polished and rusty steel electrodesmeasured with 2?40NaCl brine instead of a cement plug. Rustyelectrodes have extremely low electrical impedance.

379


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CEMENTRESISTIVITYAND IMPLICATKJNSFORMEASUREMENT PE 26453OF FORMATIONRESISTIVITYTHROUGHCASING

100000 q: E!& 10000 .01 PorewE Radius1000 cl (microns)3:IX 100 1n> 10 10!5~Q. 1?5 o,. 0 10 20 30 40 50 60 70 80 90 100

WETTING PHASE SATURATION (%)Figure 12. Capillary pressure data for cement sample 10 showing that nearly allof the porosity is present as microporosity with pore throat radii less

than 0.1 microns.

spe 26453 cement resistivity

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